6 truths about chemtrails

Recently a listener wrote to us in response to a story we aired dismissing chemtrail conspiracy theories—our listener asked us to consider both sides of the argument.

‘Chemtrails’ are a type of aircraft contrails—which is short for ‘condensation trails’, long clouds formed by water vapour in aircraft exhaust or due to pressure changes from wings.

The difference is that chemtrails are supposedly deliberately created, either to change the climate as a form of geoengineering, or some other form of weather control, or maybe mind control, or some other mass-poisoning depending on what’s being sprayed.

This is, of course, generally considered to be nonsense.

In saying that, I have considered both sides of the argument, albeit correctly weighted. As always, Lost in Science gives greater weight to peer-reviewed scientific evidence. And there is in fact a surprising amount of published research relevant to this topic.

Contrails from USAAF B-17F Flying Fortress bombers over Europe, circa 1943. That is, propellor aircraft, 72 years ago. Scientists have attempted to analyse their effect on English climate at the time. (Photo: United States Air Force)

Now, I did try to respond to our listener, but unfortunately the email bounced, so I’m writing this blog post instead in the hope they see it. Hi! Please keep listening!

And since this a largely internet-based controversy, I’ve decided to go all out for clickbait and write this in the form of a listicle. Here we go…

1. Geoengineering is a thing

Geoengineering, a popular term for deliberately manipulating the climate, is becoming a hot topic as our inadvertent climate manipulation becomes more dire. But it comes with many moral, ethical and practical risks, so it’s currently much further down the list of preferred responses than stop burning fossil fuels.

We have in fact discussed it before on Lost in Science: in March 2013 we aired a discussion on the ethics of geoengineering between Peter Singer and Clive Hamilton (author of the book Earthmasters: Playing God with the climate).

And in 2011 we looked at a range of proposed mechanisms (Geoengineering as climate change plan B), largely collated from an excellent 2009 review from the UK’s Royal Society.

2. It includes solar radiation management

This is a fancy name for reducing the amount of sunlight reaching the Earth. This is not a new idea: apart from Mr Burns, volcanoes have been doing this forever by spewing ash into the atmosphere. The eruption of Mt Pinatubo in 1991 reduced global temperatures by about 0.1 °C for a couple of years.

In fact, one of the first people to push for injecting particles—or aerosols—into the atmosphere to counteract climate change was the father of the hydrogen bomb, Edward Teller. You can listen to me talking about him at the Laborastory.

3. Aerosols emitted by humans do affect climate

For instance, sulphur pollution from China’s coal-fired power stations are believed to have contributed to the slowing of global surface temperature rise.

4. There’s no evidence anyone’s doing it on purpose

Yes, it’s hard to prove a negative. However I can direct you to the physical science working group of the 5th Assessment Report of the United Nations’ Intergovernmental Panel on Climate Change (IPCC), who said, in their Summary for Policymakers (PDF 2.3 MB):

Limited evidence precludes a comprehensive quantitative assessment of both Solar Radiation Management (SRM) and Carbon Dioxide Removal (CDR) and their impact on the climate system… Modelling indicates that SRM methods, if realizable, have the potential to substantially offset a global temperature rise, but they would also modify the global water cycle, and would not reduce ocean acidification. If SRM were terminated for any reason, there is high confidence that global surface temperatures would rise very rapidly to values consistent with the greenhouse gas forcing. CDR and SRM methods carry side effects and long-term consequences on a global scale.

It doesn’t sound to me like they’re actually doing it, or that they think it’s a good idea. And surely if it was a global conspiracy, they’d be in on it?

5. The effect of contrails has been measured, and it’s small

This is the kicker: scientists have actually studied the impact of contrails. That same IPCC report has a whole section on it (section, Contrails and Contrail-Induced Cirrus).

They find that, unlike in solar radiation management proposals, contrails actually increase the temperature, because they block the outgoing infrared (heat) radiation from the ground more than they block the incoming sunlight.

You can read the whole thing yourself (Chapter 7, PDF 19.2 MB), but I’ll save you the the trouble by quoting their conclusion:

Persistent contrails from aviation contribute a RF of +0.01 (+0.005 to +0.03) W m–2 for year 2011, and the combined contrail and contrail-cirrus ERF from aviation is assessed to be +0.05 (+0.02 to +0.15) W m–2. This forcing can be much larger regionally but there is now medium confidence that it does not produce observable regional effects on either the mean or diurnal range of surface temperature.

RF here is radiative forcing, the net thermal energy reaching the Earth. For comparison, the IPCC puts the total anthropogenic, or human-caused contribution at about 1.5 W m–2.

This is based on numerous published scientific papers. I’m not going to bore you with them here—instead you can click my Read more link and see the list. You’re welcome.

6. But aircraft emissions do have an impact overall

One of those references, Lee et al. (2009) added up the combined effect from aircraft carbon emissions and cirrus clouds caused by contrails. They found:

Total aviation RF (excluding induced cirrus) in 2005 was ~55 mW m–2, which was 3.5% of total anthropogenic forcing. Including estimates for aviation-induced cirrus RF increases the total aviation RF in 2005–78 mW m–2, which represents 4.9% of total anthropogenic forcing.

(Emphasis mine. And for readability I’ve taken out the uncertainties.)

According to the International Civil Aviation Organisation (ICAO), this 4.9% means that:

If global aviation was a country its emissions would be ranked 7th between Germany and South Korea on CO2 alone.

That’s quite a bit. So although the contribution of contrails alone is small, when you add in their carbon emissions then yes, flying planes does affect global temperature.

But even so, all this science suggests that, whatever you call them, contrails or ‘chemtrails’ are not being used as a deliberate global conspiracy.

However, we should still be concerned about the impact of air travel on the climate.


  1. Burkhardt, U., and B. Kärcher, 2009: Process-based simulation of contrail cirrus in a global climate model. J. Geophys. Res., 114, D16201.
  2. Burkhardt, U., and B. Kärcher, 2011: Global radiative forcing from contrail cirrus. Nature Clim. Change, 1, 54–58.
  3. Forster, P., et al., 2007: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Miller (eds.)] Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 129–234.
  4. Frömming, C., M. Ponater, U. Burkhardt, A. Stenke, S. Pechtl, and R. Sausen, 2011: Sensitivity of contrail coverage and contrail radiative forcing to selected key parameters. Atmos. Environ., 45, 1483–1490.
  5. Haywood, J. M., et al., 2009: A case study of the radiative forcing of persistent contrails evolving into contrail-induced cirrus. J. Geophys. Res., 114, D24201.
  6. Heymsfield, A., D. Baumgardner, P. DeMott, P. Forster, K. Gierens, and B. Kärcher, 2010: Contrail microphysics. Bull. Am. Meteor. Soc., 91, 465–472.
  7. Kalkstein, A. J., and R. C. Balling Jr, 2004: Impact of unusually clear weather on United States daily temperature range following 9/11/2001 Clim. Res., 26, 1–4.
  8. Lee, D. S., et al., 2009: Aviation and global climate change in the 21st century. Atmos. Environ., 43, 3520–3537.
  9. Ponater, M., S. Marquart, R. Sausen, and U. Schumann, 2005: On contrail climate sensitivity. Geophys. Res. Lett., 32, L10706.
  10. Rap, A., P. M. Forster, J. M. Haywood, A. Jones, and O. Boucher, 2010a: Estimating the climate impact of linear contrails using the UK Met Office climate model. Geophys. Res. Lett., 37, L20703.
  11. Rap, A., P. Forster, A. Jones, O. Boucher, J. Haywood, N. Bellouin, and R. De Leon, 2010b: Parameterization of contrails in the UK Met Office Climate Model. J. Geophys. Res., 115, D10205.
  12. Sausen, R., et al., 2005: Aviation radiative forcing in 2000: An update on IPCC (1999). Meteorol. Z., 14, 555–561.
  13. Schumann, U., and K. Graf, 2013: Aviation-induced cirrus and radiation changes at diurnal timescales. J. Geophys. Res., 118, 2404–2421.
  14. Stuber, N., and P. Forster, 2007: The impact of diurnal variations of air traffic on contrail radiative forcing. Atmos. Chem. Phys., 7, 3153–3162.

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